Families voiced their anger as hopes faded for the missing crew after the ship sank in one of South Korea's worst naval disasters. Divers tried twice to get to the wreckage, Rear Adm. Lee Ki-sik of theJoint Chiefs of Stafftold lawmakers.

The explosion at the rear of the Cheonan shut down its engine, wiped out power and caused the ship to sink a little over three hours later, the Joint Chiefs said. The exact cause was unclear, but North Korea did not appear to be to blame, officials said.

A survivor, Staff Sgt. Shin Eun-chong, 24, told relatives he was on night duty when he heard a huge boom behind him that split the ship apart. The vessel started tilting, and his glasses fell off his face as he hit the deck, relatives at a naval base inPyeongtaektold The Associated Press.

Military planes and boats were searching the waters nearSouth Korea's Baengnyeong Islandwhere the 1,200-ton Cheonan had been on a routine patrol mission. Rescue efforts Saturday were hampered by fierce waves and high winds.

Despite early fears of an attack, there was no immediate indication that North Korea — which lies within sight about 10 miles (17 kilometers) from Baengnyeong — was to blame, the Joint Chiefs said. Still, troops were maintaining "solid military readiness," Vice Defense Minister Jang Soo-man said.

Earlier, North Korea's military threatened "unpredictable strikes" against the U.S. and South Korea in anger over a report the two countries plan to prepare for possible instability in the totalitarian country.

The ship went down near a disputed maritime border that has been the site of three bloody skirmishes between the two Koreas, which remain in a state of war because their three-year conflict ended in a truce, not a peace treaty, in 1953.

President Lee Myung-bakordered officials to make all efforts to rescue the crew, spokeswoman Kim Eun-hye said.

"I can only imagine how much shock and pain the missing marines' families must be in," he said late Saturday, according to Kim.

Authorities have not confirmed any deaths, but Rear Adm. Lee told lawmakers he presumed most of the missing sailors were trapped inside, the Yonhap news agency said. Acoast guard officialsaid humans can survive in winter waters if they are rescued within two hours.

Fifty-eight of the crew of 104 were rescued, with some treated for burns, broken bones and abrasions.

Joint Chiefs spokesmanPark Seong-woo said the military will determine the cause of the accident after salvaging the vessel.

That could take weeks. In 2002, it took 17 days to salvage a 130-ton vessel struck in a surprise attack by North Korea, the Joint Chiefs said.

InPyeongtaek, south ofSeoul, wails filled the air as relatives checked a list of missing marines.

"Where are you? Where can you be?" one mother screamed.

Many relatives waited for news, their faces buried in tissues and handkerchiefs. However, about 150 people — fed up with the lack of information — pushed their way pastarmed security guardsto confront military officials. They accused authorities of a cover-up, saying survivors told them the Cheonan was leaky and in need of repair.

"Liars!" some screamed, jumping up on the car of the Cheonan's rescued captain as he tried to drive away.

The sinking is one ofSouth Korea's worst naval disasters. In 1974, a ship sank off the southeast coast in stormy weather, killing 159 sailors and coast guard personnel. In 1967, 39 sailors were killed by North Korean artillery.

"Finding the truth [behind the incident] is important, but saving our sailors is more important," Lee was quoted as saying.

A government source quoted by the South's Yonhap news agency said officials were investigating various possible causes: an attack by a North Korean torpedo boat, a mine laid by North Korea or an explosion of munitions aboard the ship.

South Korean YTN television, quoting an unidentified presidential official, said the ship was well south of the border and North Korean ships were unlikely to be in the area.

A military source told the agency the Cheonan was holed in the stern near its propeller.

War 'unlikely'

The joint chief of staff spokesman said there were no abnormal military movements on the North Korean side of the disputed maritime border, which was the scene of deadly naval clashes in 1999 and 2002.

Paul Chamberlin, a former US naval attache to South Korea, told Al Jazeera: "If it becomes clear this was an attack from North Korea, a major escalation that would lead to general war is very unlikely."

In November the navies of the two sides exchanged fire in the area. The South's officials said a North Korean patrol boat had retreated in flames but it was not known if there were any casualties. No South Koreans were hurt.

The North refuses to accept the maritime border known as the Northern Limit Line, which was drawn up by UN forces after the 1950-53 Korean war. It says the line should run further to the south.

In January, the North fired 370 artillery shells into the sea near the border, raising tensions between the two sides.